fix(fileutils): accept os.PathLike in AtomicSaver

[uttam12331]

AtomicSaver.__init__ assigned dest_path directly to self.dest_path and later concatenated it as a string (dest_path + '.part'), which raises TypeError when a pathlib.Path is passed.

iter_find_files called .split() on the directory argument, which also fails for pathlib.Path.

Changes:

• AtomicSaver.__init__: convert dest_path via os.fspath() at the top of the method, and use self.dest_path (the already-converted str) when building the .part path
• iter_find_files: convert directory via os.fspath() before calling .split()
• Add test_atomicsaver_pathlike and test_iter_find_files_pathlike to the test suite

Closes #383

[mahmoud]

Good call, pathlib support here was overdue. Thanks for the fix + tests!